Selective corneal aberrometry

ABSTRACT

A method and system are disclosed for measuring and mapping the anterior surface topography of a cornea to determine corneal aberrations and an optimal ablation pattern for refractive and therapeutic surgery of the cornea The invention stimulates a prospective ablation process with real-time visual feedback accurately portraying results of topographic surface alter-ation that would occur during an actual ablation procedure, thereby achieving minimal conreal aberrations and optimal image quality.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to surface profiling, andmore specifically, to a method and system for measuring and mapping theanterior surface topography of a cornea to determine corneal aberrationsand an optimal ablation pattern for refractive and therapeutic surgeryof the cornea.

[0003] 2. Description of the Related Art

[0004] Various methods have been developed to reshape the cornea of thehuman eye in order to correct for vision defects. Among these visiondefects are nearsightedness (myopia), with the unaccommodated nominalfocusing plane falling before the retina; farsightedness (hyperopia),with focusing plane beyond the retina; and the combination of defectsknown as astigmatism, in which the cornea has a toroidal shape and thereis no plane of best focus. The most common methods to correct for thesedefects are spectacles and contact lenses (hard, soft and gas permeabletypes) that provide the correct amount of refractive power to shift theunaccommodated focusing plane to its optimum position on the retina.However, glasses are worn externally and are not infrequently perceivedto be uncomfortable, inconvenient, or detracting from personalappearance.

[0005] Contact lenses are sometimes utilized when the use of glasses hasbeen considered to be undesirable, mostly for cosmetic reasons. However,contact lenses entail problems of their own in terms of possible eyeinfection and the necessity for time consuming procedures required tomaintain sterility and minimize contamination. More importantly, manypeople cannot tolerate the insertion of foreign objects on or in theireyes.

[0006] In response to a need for safe permanent correction of vision,without recourse to glasses or contact lenses, several major surgicalmethods for vision correction have evolved. For instance, radialkeratotomy (RK), involves surgical incision of the cornea, with deepradial cuts outside the vision zone that cause a roughly predictableflattening of the cornea and a reduction in refractive power thereof,suitable for correcting low levels of myopia. Another procedure iscorneal ablation with an excimer laser (photo refractive keratotomy(PRK)) which is achieved by selectively ablating corneal tissue from theanterior surface of the cornea or by varying the front surface curvatureof the cornea.

[0007] Before corrective surgery, a patient usually has severaldiagnostic tests to determine the shape of the corneal surface. To date,measuring the corneal surface involves the use of aberrometric devicesthat generate an individual diagnostic set of data relating to the eyestructure to determine an ablation pattern for subsequent adaptation toan appropriate laser delivery system. However, systems and methods usedheretofore determine total aberrations of the optical system andintroduce not only corneal aberrations but also aberrations relating tovariable elements within the eye, such as the crystalline lens,accommodation of the lens, and vitreous structures. Thus, the measureddioptric values do not provide the option of differentiating aberrationscaused by the variable elements from that of corneal aberrationsoriginating on the anterior corneal surface. As such, a correctiveablation procedure for reshaping the cornea may not adequatelycompensate or may over-compensate due to aberration data from theundifferentiated variables.

[0008] Further, determining the topographic surface of the cornea withtoday's aberrometers does not provide sufficient data to measure theentire area of the cornea because generally the aberrometers utilizeonly around 100 measurement points with an accuracy of±100 μm in the xand y plane. Considering that the x/y reference plane is centered aboutthe pupilar margin, and a corneal surface is usually about 8.5 mm indiameter, there is not sufficient data to generate a topographic surfacemap with the necessary precession.

[0009] Accordingly, at the present time, a need exists in the art for amethod and system that determine the topographic surface of the corneawith sufficient accuracy.

SUMMARY OF THE INVENTION

[0010] The present invention relates to a method and system fordetermining the surface topography of a cornea to accurately representthe topography of the corneal surface and provide guidance for permanentcorneal reshaping that corrects for only corneal aberrations.

[0011] Additionally, the present invention relates to a system andmethod that provide a virtual image of the topographic surface of thecornea that may be combined with virtual components of the opticalsystem to determine the extent of corneal ablation required to achieve apositive effect on the optical properties of the eye.

[0012] Further, the present invention relates to a system and methodthat provide simulation of a prospective ablation process with real-timevisual feedback accurately portraying results of topographic surfacealteration that would occur during an actual ablation procedure toachieve minimal corneal aberrations and optimal image quality.

[0013] Still further, the present invention relates to a system andmethod that provide an accurate determination of corneal aberrationsthat do not include aberrations caused by internal structures within theoptical system, i.e., crystalline lens, aqueous and vitreous humor.

[0014] In one aspect, the present invention relates to a system foranalyzing optical properties of a structure, the system comprising:

[0015] means to topographically measure a surface of the structure andproduce a measured topography of the surface;

[0016] means to create a virtual surface corresponding to the measuredtopography of the surface;

[0017] means to calculate paths of generated light beams contacting thevirtual surface; and

[0018] means to analyze the calculated paths.

[0019] In another aspect, the present invention relates to a system foranalyzing optical properties of a structure into which light waves enterthrough a surface of the structure, the system comprising:

[0020] means to topographically measure a surface of the structure andproduce a measured topography of the surface;

[0021] means to create and display a virtual structure substantiallycorresponding to the structure;

[0022] means to create and display a virtual surface corresponding tothe measured topography of the surface of the structure, wherein thevirtual surface is combined with the virtual structure;

[0023] means to generate and display virtual light waves for passagethrough the virtual surface into the virtual structure;

[0024] means to calculate paths of the virtual light waves passingthrough the virtual surface; and

[0025] means to analyze the calculated paths to determine refractivepower of the virtual surface.

[0026] In still another aspect, the invention relates to a system foranalyzing optical properties of a structure into which light waves enterthrough a surface of the structure, for resolution of the light waves tocreate a picture, the system comprising:

[0027] means to topographically measure a surface of the structure andproduce a measured topography of the surface;

[0028] means to create and display a virtual structure substantiallycorresponding to the structure;

[0029] means to create and display a virtual surface corresponding tothe measured topography of the surface of the structure, wherein thevirtual surface is combined with the virtual structure;

[0030] means for altering the virtual surface;

[0031] means to generate and display virtual light waves for passagethrough the virtual surface into the virtual structure;

[0032] means to calculate paths of the virtual light waves passingthrough the virtual surface; and

[0033] means to analyze the calculated paths to determine when thevirtual surface has been sufficiently altered to provide a refractivepower that shifts the virtual light waves to a position within thevirtual structure for resolution of the virtual light waves.

[0034] In a further aspect, the invention relates to a method foranalyzing optical properties of a structure into which light wavesenters through a surface of the structure, the method comprising:

[0035] measuring topographically a surface of the structure to produce ameasured topography of the surface;

[0036] creating and displaying a virtual structure substantiallycorresponding to the structure;

[0037] creating and displaying a virtual surface corresponding to themeasured topography of the surface of the structure and combining thevirtual surface with the virtual structure;

[0038] generating and displaying virtual light waves for passage throughthe virtual surface into the virtual structure;

[0039] calculating paths of the virtual light waves passing through thevirtual surface; and

[0040] analyzing the calculated paths to determine refractive power ofthe virtual surface.

[0041] A further aspect of the invention relates to a method foranalyzing optical properties of a structure into which light waves enterthrough a surface of the structure, for resolution of the light waves tocreate a picture, the method comprising:

[0042] measuring topographically a surface of the structure to produce ameasured topography of the surface;

[0043] creating and displaying a virtual structure substantiallycorresponding to the structure;

[0044] creating and displaying a virtual surface corresponding to themeasured topography of the surface of the structure, wherein the virtualsurface is combined with the virtual structure;

[0045] altering the topography of the virtual surface;

[0046] generating and displaying virtual light waves for passage throughthe altered virtual surface into the virtual structure;

[0047] calculating paths of the virtual light waves passing through thealtered virtual surface; and

[0048] analyzing the calculated paths to determine when the alteredvirtual surface has been altered sufficiently to provide a refractivepower that shifts the virtual light waves to a position within thevirtual structure for optimal resolution of the virtual light waves toform a picture.

[0049] In a still further aspect, the present invention relates to amethod for analyzing optical properties of a structure, the methodcomprising:

[0050] measuring topographically a surface of the structure andproducing a measured topography of the surface;

[0051] creating a virtual surface corresponding to the measuredtopography of the surface;

[0052] calculating paths of generated light beams contacting the virtualsurface; and

[0053] analyzing the calculated paths.

[0054] Therefore, the invention provides a method and a system thatdetermine the topographic surface of a cornea without necessarilycombining aberration data received from other structures within theoptical system and that provide a virtual simulation model utilizing thedetermined topographic surface for simulating results of a prospectiveablation procedure.

[0055] It is obvious that a visual display of the calculated surfaces,light beams or the like might be omitted as long as the necessary dataare provided to a person using the method or the system according tothis invention.

[0056] Obviously, this method and this system are not limited tomeasurements and ablation processes at an eye but may be used with anyoptical system focusing pictures in a body, which pictures are createdby beams entering said body through a surface. Especially, this methodand this system may be used if the body comprises an internal structurebeing optically active.

[0057] Other aspects, features and embodiments in the invention will bemore fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058]FIG. 1 shows the reflection pattern of a projection patterncomprised of several concentric rings projected on a spherical surface.

[0059]FIG. 2 shows an example of a comparable reflection pattern in theevent of a cornea deviating from the spherical configuration.

[0060]FIG. 3 shows a schematic illustration of an arrangement forcarrying out the process according to the invention.

[0061]FIG. 4 is a block diagram of the system components of a preferredembodiment.

[0062]FIG. 5 shows a quality surface map according to the presentinvention.

[0063]FIG. 6 shows a corneal aberration map according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

[0064] Determining pre-operative corneal topographic abnormalities is amajor determinant for ensuring predictability and stability of asubsequent refractive surgical procedure. The present invention is basedon the discovery that determining and utilizing the corneal surfacetopography is sufficient to analyze the optical system's deficienciesand provide information on the predictability and stability of thesesurgical procedures. Moreover, the present method and system enable thesimulation of an ablation procedure to determine an effective customablation pattern to produce a positive effect on the optical propertiesof the eye resulting in sufficient refractive power to shift anunaccommodated focusing plane to its optimum position on the retina forincreased visual acuity.

[0065] As stated above, the human eye is an optical system characterizedby aberrations of several types. A contributing factor to theseaberrations is the fact that the human eye is not a rotationallysymmetric optical system. Additionally, corneas are never perfectlyspherical, and as such, it is important that the deviations therefrom beaccurately and topographically pre-determined for effective refractivecorrection.

[0066] Corneal topography is a method of measuring and quantifying theshape and curvature of the corneal surface. Most topographic devicesinclude a placido disc made up of multiple circles, which is back lit orprojected onto the corneal surface. The resultant circular images arereflected and captured with a video camera and then digitized forsubsequent display on a monitor. Using the mathematics of convex mirrorsand mathematical algorithms, the image size is measured and quantified.Generally, any conventional corneal topography device may be utilized inthe present invention to generate illuminated concentric rings that areprojected onto the anterior corneal surface of a patient's eye. Theemitted light rays are reflected off the patient's cornea and at least aportion of the reflected light rays are captured by a lens and focusedonto an imagining system, such as the video camera.

[0067] In accordance with the present invention, a preferred topographicinstrument is the Tubingen Colour Ellipsoid Topometer (C-Scan)commercially available from Technomed Technology, Germany, as describedin U.S. Pat. No. 5,640,962, the contents of which are incorporated byreference herein, for all purposes. Specifically, the topographicinstrument provides x, y and z data to form a 3-D elevation map byutilizing at least three distinguishable recognition marks within aprojection pattern projected onto the cornea surface. Thedistinguishable markings are utilized for measuring the surfacetopography of the cornea and accomplished by a projection body which isilluminated, for example, by a white light source and has transparent,preferably ring shaped zones of different colors. In the alternative, itpossible to provide an arrangement of differently colored light sources,for example in form of an array of diodes.

[0068] Preferably, a graphics processing unit is utilized to capture thereflected data to form an image representing the reflective surfacewhich may be compared with a stored reference, or other knowninformation, to identify any distortions in the captured image. Thecomputer means measures the deflection of the light rings by thereflecting anterior corneal surface. Specifically, the distortion oflight reflected through the ring pattern by the conditions that exist onthe corneal surface is analyzed to determine the corneal topography andany deformation in the patient's cornea. The resulting data may bedisplayed as a corneal curvature map wherein different colors correspondto corneal power and curvature.

[0069]FIG. 1 shows schematically the image of an eye 1 of configuration2 as registered by an image detection system of the above-describedtype. Depicted within the interior of the circumferential line 2 is thecontour of the cornea 3. A placido disc is used to reflect rings ofdifferent colors onto the surface of the eye. Preferably, severalreflection rings 4 to 10 are generated by the cone device shown in FIG.3, wherein the rings are concentric to the Z-axis of the optical axis(camera-corneal apex). Located within the interior of these rings aretwo centering objects 11 and 12 that are described in more detailfurther below. The distance as well as the concentric and circulararrangement of the rings 4 to 10 correspond to the reflected image of ahealthy cornea with spherical surface.

[0070] In the case of astigmatism, the corresponding image of a deformedcornea is shown in FIG. 2. The structures 4′ to 8′ represent also amirror reflection pattern of absolute concentric and circular projectionrings. Their image is deformed by a non-spherical corneal surface. Inpart, the images 5′ and 6′ of the respective and originally closedprojection rings have gaps 13, 14, 13′ and 14′ while other structures 7′and 8′ show significant dents 15 and 16. In a normal black/whitephotograph, which cannot be illustrated in the drawing any differently,the zone 15 of the structure 7′ cannot unequivocally be associated to arespective projection ring. This zone 15 could be associated to theprojection rings as formed by the reflection rings 5, 6 or 7 of ahealthy cornea (FIG. 1). Advantageously, because the rings 4 to 10 andthe structures 4′ to 8′ are colored by a respective coloration of theprojection pattern the zone 15 can be unequivocally associated with thecorrect projection ring.

[0071] In a preferred embodiment, a projection body in the form of ahollow cone or a hollow ellipsoid with transparent rings, in particularcolor rings, is used in the side wall. FIG. 3 shows a schematic,sectional illustration of an arrangement for carrying out the measuringprocess according to the present invention. Positioned in front of theeye 1, with the arched cornea 3, is a projection body 17. The projectionbody 17 includes a cone-shaped hollow body, with a side wall 18 havingring-shaped, transparent and differently colored passages 19. Thecone-shaped projection body 17 is illuminated from outside by aring-shaped neon tube 20. The showing of two light beams emitting fromdifferently colored ring-shaped passages 21 and 22 and symbolized bylines 23 and 34 illustrates the projection of the ring structures uponthe cornea. The beams 23′ and 24′ reflected from the cornea 3 radiatethrough a pinhole diaphragm at the narrower end of the cone-shapedprojection body 17 and form an image in an image detector 26. The Z-axisis formed, as stated above, by the prolongation of the axis of the imagedetection unit relative to the surface being measured.

[0072] It is important in the described process to attain a goodadjustment of the surface, for example the corneal surface, relative tothe image detection unit in Z-direction. The image-forming qualities ofthe entire arrangement depend significantly on this distance. Theadjustment in direction of the Z-axis is preferably carried out throughscanning-in of at least two centering objects in the projection patternat a particular angle between their respective projection axes. Theintersection of both projection axes yields the desired correctZ-position.

[0073] Centering objects 11 and 12 are scanned in at a certain anglebetween their projection axes in a plane which coincides with theZ-axis.

[0074] Alignment of objects 11 and 12 provides for good adjustment ofthe cornea surface relative to the image detection unit in theZ-direction and allows the determination of the Z-position of thecorneal surface 3 and its evaluation. Laser beam 28 forms the centeringobject 11 e.g., on the cornea and laser unit 27 is arranged outside theprojection body 17. The projection body includes a small aperture in itswall for passage of the laser beam 28 which then intersects the Z-axisin the desired point.

[0075] Preferably, the image detector 26 includes a video cameracommunicatively connected to a graphics processing unit. The reflectionsignals from the corneal surface are converted into electrical signalsrelative to their intensity and the signal may be automatically executedin the graphics processing unit having a display monitor. Preferably adigital camera is utilized because the electronic signals resulting fromthe charge coupled devices (CCD) are not turned into an analog signal,but instead remain in the form which they are recorded and transferredto the memory of the graphics processing unit as digital signals. Theresults may be outputted in the form of isoreflection lines which referto those areas of the surface that have a same reflective power duringthe described measurement. Thus, the deviations from a standard ofmeasurement are visible directly and without requiring any furtherinterpretation of the measuring data.

[0076] To optimize the mapping of small corneal irregularities incorneal topography for subsequent use in corneal and refractive surgery,the data of small corneal irregularities are displayed in 3-D elevationmaps.

[0077] Amplification is needed to display those irregularities e.g.,scars, central island after PRK, amount of tissue removed. This may beachieved by subtracting a best fit sphere or in the alternative, asoftware program, commercially available from C-Scan, TechnomedTechnology, Germany, may be utilized. The program specifically utilizesthe line of sight as a reference point (Z axis) combined with vectoranalysis and provides a multiplicity of point evalutions ranging fromabout 100 to 1000 to improve resolution. Further, this software tool iscapable of compensating for errors which occur due to different imageplanes of different video topography pictures, such as before or afteran ablation procedure.

[0078] The resolution of the corneal map cannot, in general, exceed thenumber of positions at which the data was acquired. It will beappreciated that increasing the resolution of the data can enhance therepresentation of the cornea scanning. To increase the resolution of thetopographic surface of the cornea, a method described in U.S. Pat. No.5,900,924 may be implemented, the contents of which are incorporated byreference herein for all purposes.

[0079] The generated high resolution topographic 3-D map of the cornealsurface is displayed on a graphics processing unit. To determine optimalcustom ablation of the measured topographic surface, a virtualstructure, such as an eye structure is created on a geometric surface toform a manipulative three dimensional model of the eye and displayed onthe graphics processing unit. The virtual eye may be created by anymethod known in the art including, photographing components of an idealeye and texture mapping the photograph to develop a mathematical modelof the eye. Further, an ideal virtual structure may be generated by avirtual reality drawing program such as, Optics Lab™, available fromScience Lab Software, Carlsbad Calif. Additionally, a virtual retina iscreated to provide a surface for positioning a virtual image formed byconvergence of light waves.

[0080] After creation of a virtual ideal eye structure, a virtual imageor the image of the calculated topographic surface is connected orsuperimposed on the virtual ideal eye structure and displayed on thegraphics processing unit. Additionally, internal structures of the eyestructure may be included within the ideal virtual eye structure. Forexample, the patient's iris may be mathematical generated by a raytracing virtual reality program or photographed for digital recreationon the displaying monitor. A digitized system is commercially availablefrom Iridian Technologies, Moorestown, N.J.

[0081] Accordingly to the method and system of present invention, theoptical properties of the topographical surface may be determined by thegeneration of virtual light waves that contact and/or pass through thevirtual topographic surface into the interior of the ideal eyestructure. A number of ray tracers software programs are currentlyavailable that convert a graphics processing unit or computer into anoptics workstation and generate different beam platforms for design andanalysis of optics system including, Optica™ which is commerciallyavailable form Wolfram Research, Inc., and Stellar™ from StellarSoftware, Berkeley, Calif.

[0082] The generated virtual light waves may be in several forms, suchas parallel beams, beams having varying intensity or color, patterns andfrequencies.

[0083] Based upon data generated by the 3-D elevation map created bymethods discussed above, a corneal surface segment which is defined byan area between at least four (4)measured points is tested for visualacuity. Fast ray tracing, using the high resolution 3-D elevation datain conjunction with Snell's law may be used to describe the diffractionof the incident virtual light waves and the resulting images on thevirtual retina. Generally, every measurable surface irregularity causesa deviation for an incident light beam (optical aberration) which can bedetected and quantified. A virtual light beam passing through themeasured corneal surface, the iris and projected into a virtual retinaresults in an offset in relation to an ideal beam, passing through anoptically ideal corneal surface. This offset can be processedquantitatively by projecting two points on the corneal surface whereby amathematical peak to valley analysis leads to an index for the cornealresolution. The index values are determine for a multiplicity of cornealsegments, preferably at least 100 segments, and more preferably between250 and 1000 segments, to determine an index for the best correctedcorneal visual acuity and deviations therefrom.

[0084] Accordingly, the index values are correlated and projected backonto the corneal surface to generate a surface quality map. The surfacequality map identifies the areas of the cornea with good or poor opticalquality (lower or higher corneal aberration) and can display thisinformation in both percentages and/or a color coded map as shown inFIG. 5. This surface quality map can also be used to determine thefunctional optical zones which show relative good optical quality andthe map may be color coded according to visual acuity of more than20/20.

[0085] Further, subtracting a best fitted sphere or asphere from themeasured topographically surface will generate a difference map showingthe amount and the location on the x and y plane of the cornea tissuethat has to be removed during surgery to achieve a surface topographythat improves visual acuity. Thus, the corneal surface is corrected byremoving only tissue that will effect and improve visual acuity andavoiding ablation of corneal tissue that does not contribute to theimprovement of optical properties.

[0086] Further, data relating to contrast sensitivity may be generatedby varying the frequencies of the virtual light waves (sine or cosinewave) to calculate a projection that is comparable to known clinicalcontrast sensitivity charts. Thus, the high resolution 3-D topographicmap may be further utilized as a projection screen whereon individualcorneal segments are analyzed for a modular transfer function (MTF) andphase shift function (PSF) to quantitatively generate a optical transferfunction. (Both the MTF and PSF may be created by the optics systemOptica™ described above)

[0087] Heretofore, clinically used contrast sensitivity represented acombination of the contrast (modular transfer function) and sensitivitywhich is a function of the retina. Both parameters are measured in oneclinical examination and subsequently included in a known vector visionchart. However, this is a subjective measurement and does not provideisolated information on the corneal surface unless a pre- andpost-operative measurement is taken. This may still not always beeffective because the patient must be extremely cooperative on bothexaminations to insure continuity in the alignment of the optical axis.

[0088] By the methods of the present invention the modular transferfunction MTF appears as contrast and the phase shift function PSFindicates the shift of the image relative to the optical axis (Z-axis).Basically, a contrast map can be generated by using the 3-D elevationdata of the video topometer and a simulated projection of a virtualsinus wave. Practically, this can be accomplished by using a pointspread function and the projection of two points on the surface. Usingthe resulting point spread function g(k), the convolution F_(s)(x) iscalculated using the equation:

F _(s)(x)=½Σ_(k)(1+cos[α{x-k}])g(k)⁶

[0089] with k=shift of spread function and x=local position on virtualretina.

[0090] The calculated projection of the sinus wave using real cornealelevation data can be analyzed for the contrast behavior C using theequation:

C=(I _(max) −I _(min))/(I _(max) +I _(min))²

[0091] Where I_(max)=maximal light intensity (before passing cornea) andI_(min)=minimal light intensity (intensity on vitual retina).

[0092] A resulting graph or output, shown in FIG. 6, illustrates thecontrast (modular transfer function) as a function of the sinus wavewherein visual acuity is given by the intersection of the contrast curvewith the retinal sensitivity line.

[0093] The phase shift of the image is calculated to determine localtilt of the corneal surface for determining minimal amounts ofdecentration due to ablation procedures. Decentration acts as a localtilt and therefore leads to a shift of the image with regard to theoptical axis. Different frequencies of sinus waves are transferred tothe virtual retina, which is defined as the focal plane resulting fromapproximately three millimeters within the central area of the cornea.The phase shift occurs when the graph of a sin function A sin(ωt+φ)crosses the x axis and may be calculated by using a trigonometricidentity for the sums of angles wherein the sum of a sine and a cosinecurve is equivalent to a sine curve with a phase shift. A second graphshows the phase shift of the projected sinus wave relative to theoptical axis as a function of the frequency. Both graphs, together withthe included information, provides an optical transfer function (OTF).Further, the two graphs may be combined with the surface quality map,which provide the optical quality of each corneal point, to display thecorneal optical aberrations as shown in FIG. 6.

EXAMPLE I

[0094] A standard 4.5D PRK myopia correction with an optical zonediameter of 6.5 mm, treated with an Apex plus (Summit, USA), was used toillustrate the obtained corneal aberration map (CAM), shown in FIG. 6,which combines the two graphs and surface quality map of FIG. 5. Theaberration map is divided into four sub charts including, a radius map,surface quality map, contrast graph and phase shift graph. At the lowerleft of the CAM, the distribution of radii is shown which indicates ahomogenous distribution. In the upper left of the CAM, the surfacequality map shows a central dark gray area (normally a colored amp)which indicates, quantitatively, the optical aberration prevailing in anormal 20/20 eye having a diameter of 4.2 mm after the 4.5D PRK. Theupper right graph shows “contrast” as a function of sinus wavefrequencies and indicates that contrast decreases rapidly for lowerfrequencies. The horizontal line represent, in this graph, thephysiological threshold and the intersection between the contrast curveand the physiological threshold correlates to the expected potentialvisual acuity for sinus wave structures. The phase shift graph in thelower right of the CAM shows the behavior of the phase shift as afunction of frequencies and indicates the phase shift is almostconstant.

[0095] The present map with the four charts can be used to provideinformation necessary to qualify the corneal aberration. While thecorneal radius map is homogenous for the 6.5 mm diameter of the ablatedoptical zone, the surface quality map reduces the functional opticalzone to 4.2 mm for a given amount of myopic correction. Thus, fordaylight conditions, a functional optical zone of 4.2 mm in connectionwith a small pupil, provide high visual acuity and good contrastbehavior. The information of the corneal aberration map can be used toguide a laser for customized ablation. Moreover, the corneal aberrationmap contains all the information to achieve minimal corneal aberration,and thus, optimal image quality.

[0096] Another embodiment of the present invention relates to themanipulation of the corneal surface to be performed by a virtualaltering instrument created within the graphics processing unit orcomputer. For a virtual ablation procedure, a cut or removal of tissuecaused by the virtual instrument is monitored by tracking the tip of theinstrument and the virtual surface is reshaped to correspond to thevirtual tissue removal. Subsequently, the visual acuity, as a result ofeach ablation cut, may be determined by testing the altered virtualtopographic surface of the virtual cornea by methods described above.Accordingly, a pre-operative test may be performed to determine theoptimal ablation procedure and to determine and achieve minimal cornealaberration.

[0097] In case of a medical application, in particular for determiningthe surface topography of the corneal surface of an eye, a measuringdevice which operates according the stated process, can be coupleddirectly onto the operating microscope to enable the surgeon torecognize corneal areas having a deviation from the desired geometry andto immediately render respective treatment.

That which is claimed is:
 1. A system for analyzing optical propertiesof a structure into which light waves enter through a surface of thestructure, the system comprising: means to topographically measure asurface of the structure and produce a measured topography of thesurface; means to create a virtual structure substantially correspondingto the structure; means to create a virtual surface corresponding to themeasured topography of the surface of the structure, wherein the virtualsurface is combined with the virtual structure; means to generatevirtual light waves for passage through the virtual surface into thevirtual structure; means to calculate paths of the virtual light wavespassing through the virtual surface; and means to analyze the calculatedpaths to determine refractive power of the virtual surface.
 2. Thesystem according to claim 1, wherein the structure is an eye having atopographic surface.
 3. The system according to claim 1 or 2, furthercomprising means to measure an internal structure within the structureand means to create a virtual internal structure substantiallycorresponding to the internal structure.
 4. The system according toclaim 3, wherein the internal structure is an iris.
 5. The systemaccording to anyone of claims 1 to 3, wherein said means to analyze thecalculated paths to determine the refractive power of the virtualsurface, comprise means to create a virtual focal area, i.e. a virtualretina.
 6. The system according to claim 5, wherein the calculated pathsof the light waves merge at the focal area.
 7. The system according toanyone of claims 1 to 6, wherein the light waves have varying intensity.8. The system according to claim 7, wherein the means to analyze thecalculated paths comprise means to analyze the phase shift of the beamsof varying intensity, passing through the virtual surface, wherein thephase shift is analyzed with respect to an optical axis of thestructure.
 9. A system for analyzing optical properties of a structureinto which light waves enter through a surface of the structure forresolution of the light waves to create a picture, the systemcomprising: means to topographically measure a surface of the structureand produce a measured topography of the surface; means to create avirtual structure substantially corresponding to the structure; means tocreate a virtual surface corresponding to the measured topography of thesurface of the structure, wherein the virtual surface is combined withthe virtual structure; means for altering the virtual surface; means togenerate virtual light waves for passage through the virtual surfaceinto the virtual structure; means to calculate paths of the virtuallight waves passing through the virtual surface; and means to analyzethe calculated paths to determine when the virtual surface has beensufficiently altered to provide a refractive power that shifts thevirtual light waves to a position within the virtual structure forresolution of the virtual light waves.
 10. The system according to claim9, wherein the structure is an eye having a topographic surface.
 11. Thesystem according to claim 9 or 10, further comprising means to measurean internal structure within the structure and means to create a virtualinternal structure substantially corresponding to the internal structureand being positioned in the virtual structure behind the virtualsurface.
 12. The system according to claim 11, wherein the internalstructure is an iris.
 13. The system according to anyone of claims 9 to11, wherein said means to analyze the calculated paths to determine whenthe virtual surface has been sufficiently altered, comprise means tocreate a virtual focal area, i.e. a virtual retina.
 14. The systemaccording to claim 13, wherein the calculated paths of the virtual lightwaves merge at the focal area.
 15. The system according to anyone ofclaims 9 to 14, wherein the light waves have varying intensity.
 16. Thesystem according to claim 15, wherein the means to analyze thecalculated paths comprise means to analyze the phase shift of the beamsof varying intensity, passing through the virtual surface and the phaseshift is analyzed with respect to an optical axis of the structure. 17.A system for analyzing optical properties of a structure, the systemcomprising: means to topographically measure a surface of the structureand produce a measured topography of the surface; means to create avirtual surface corresponding to the measured topography of the surface;means to calculate paths of generated light beams contacting the virtualsurface; and means to analyze the calculated paths.
 18. A method foranalyzing optical properties of a structure, the method comprising:measuring topographically a surface of the structure and producing ameasured topography of the surface; creating a virtual surfacecorresponding to the measured topography of the surface; calculatingpaths of generated light beams contacting the virtual surface; andanalyzing the calculated paths.
 19. A method for analyzing opticalproperties of a structure into which light waves enter through a surfaceof the structure, the method comprising: measuring topographically asurface of the structure to produce a measured topography of thesurface; creating a virtual structure substantially corresponding to thestructure; creating a virtual surface corresponding to the measuredtopography of the surface of the structure and combining the virtualsurface with the virtual structure; generating virtual light waves forpassage through the virtual surface into the virtual structure;calculating paths of the virtual light waves passing through the virtualsurface; and analyzing the calculated paths to determine refractivepower of the virtual surface.
 20. The method according to claim 19,wherein the structure is an eye having a topographic surface.
 21. Themethod according to claim 19 or 20, further comprising means to measurean internal structure within the structure and means to create a virtualinternal structure substantially corresponding to the internal structureand positioned in the virtual structure behind the virtual surface. 22.The method system according to claim 21, wherein the internal structureis an iris.
 23. The method according to anyone of claims 19 to 22,creating a virtual focal area, i.e. a virtual retina.
 24. The methodaccording to claim 23, wherein the calculated paths of the virtual lightwaves merge at the focal area.
 25. The method according to anyone ofclaims 19 to 24, wherein the light waves have varying intensity.
 26. Themethod according to claim 25, wherein the means to analyze thecalculated paths comprise means to analyze the phase shift of the beamsof varying intensity, passing through the virtual surface wherein thephase shift is analyzed with respect to an optical axis of thestructure.
 27. A method for analyzing optical properties of a structureinto which light waves enter through a surface of the structure forresolution of the light waves to create a picture, the methodcomprising: measuring topographically a surface of the structure toproduce a measured topography of the surface; creating a virtualstructure substantially corresponding to the structure; creating avirtual surface corresponding to the measured topography of the surfaceof the structure, wherein the virtual surface is combined with thevirtual structure; altering the topography of the virtual surface;generating virtual light waves for passage through the altered virtualsurface into the virtual structure; calculating paths of the virtuallight waves passing through the altered virtual surface; and analyzingthe calculated paths to determine when the altered virtual surface hasbeen altered sufficiently to provide a refractive power that shifts thevirtal light waves to a position within the virtual structure forresolution of the virtual light waves.